At the end of the first Diablo game, the hero pushes Diablo's soulstone in his forehead.
If I recall it correctly(many years have passed since I played Diablo) he did that "to contain its evilness".
Was it really necessary, especially given the fact that there is an effective way to destroy the soulstones? Or was it just a small part of Diablo's bigger plan?

2 Answers
2

The Soulstones were created by Tyrael to contain each of the Prime Evils - Baal, Mephisto, and Diablo. It seems the original thought was to contain their souls in the Soulstones, so "the hero" was following the plan in Diablo I. You can tell, because of the lore regarding the Binding of Baal (from the Diablo II manual).

Weakened by his exertions, Baal let loose one final strike against Tal Rasha; yet, thankfully, the mage was left relatively unhurt. Unfortunately, the sacred Soulstone that he had been given by the Archangel Tyrael was shattered into several small pieces. Reeling in panic, we pressed our attack and succeeded in temporarily subduing the raging demon.

Knowing that the Soulstone's broken shards would not be enough to contain Baal's powerful essence, Tal Rasha quickly devised a reckless plan to contain the demon forever. With a feverish light in his eyes, he coldly walked over to Baal's writhing form and slit the creature's throat. As Baal's spirit fled the dying body, Tal Rasha chose the largest of the Soulstone's shards and jammed it into the open wound. Just as with Mephisto, Baal spirit was sucked into the golden shard's vacuous recesses and trapped. The shard pulsed and hummed as though unable to hold its terrible content in check. Though we questioned his judgement, Tal Rasha seemed confident that the shard would hold Baal until our task was complete.

So, you can see that Tal Rasha did the exact same thing with Baal, even with accomplices. The main differences being that the Baal's soulstone was damaged, and Tal Rasha's posse knew it wasn't going to hold him. "The hero" thought the Soulstone would hold Diablo. What he didn't know was that the three Prime Evils had been taught how to corrupt the Soulstones by a lesser evil. You learn this as the "new hero" in Diablo II. Per wikipedia:

The player learns of the truth behind the corruption and the story of the soulstones. Diablo released Mephisto (Lord of Hatred) and Baal (Lord of Destruction) from their soulstones, as they were taught long ago how to corrupt them by the fallen angel Izual...

The sacred Soulstones were given to the Horadrim by the Archangel Tyrael in order to imprison the Prime Evils that had been set loose upon the world. However, the fallen angel Izual had told the Prime Evils how to corrupt the sacred stones and bend their tremendous power to their will. By doing so the Soulstones no longer acted as a prison but instead strengthened the Evils beyond the boundaries of the realms and even functioned as an anchor into the mortal realm: if the corporeal forms of the Prime Evils were destroyed their spirits remained within the stones, making a rebirth into the world possible.

That sucks, doesn't it? So you can't really fault "the hero". It seems he was just playing from Tyrael's playbook and got the short end of the stick. It wasn't until Diablo II where the "new heroes" learned that the corrupted Soulstones could be destroyed at the HellForge.

When I first saw that cut scene, I thought that it was a combination of arrogance and greed. Something along the lines of "I can contain and channel this power without being overwhelmed by evil!" 15 year old me thought that the result was inevitable but that the reasoning made a lot of sense.

So, to answer your question: Yes, it was unavoidable. Someone has to count all that money, and Blizzard decided they were the folks for the job.